Aqueous based working fluids encompass a broad spectrum of liquids, for example, machining fluids, hydraulic fluids, coolants and heat transfer fluids. The aqueous heat transfer fluids are widely used in boiler and chiller systems and are typically treated with one or more additives to improve the performance of the fluid and/or protect the wetted components of the system including pipes, valves, pumps and heat transfer surfaces. Organic lubricants may be used to reduce friction and heat production and reduce or prevent wear of contacting parts. Corrosion inhibitors may be used to reduce or prevent metallic, or in some cases non-metallic, corrosion or other degradation of the system components.
Biocides and fungicides may be used reduce or prevent microbial or fungal growth within the system. Other additives may be used to prevent or reduce foaming, precipitation of metal contaminants, scale formation or misting. Aqueous systems also frequently include one or more surfactants (i.e., surface active agents) that may contribute to the system lubrication and/or maintaining a stable suspension of water insoluble particles within the working fluid.
The properties of aqueous working fluid compositions, particularly those in boiler systems operating under high cycles of concentration (also referred to as COC or Cycles) are known to change both qualitatively and quantitatively over a period of use. This is particularly true for systems with severe operating conditions (e.g., high temperatures, high solids, and high shear forces). These changes may be the result of one or more factors such as evaporation, reactions (e.g., oxidation or corrosion), thermal degradation and physical degradation of one or more components of the aqueous working fluid composition or the system equipment.
Controlling, preventing and/or compensating for the anticipated chemical and/or physical changes in aqueous working fluids and/or the additive package during use is important to the economic and functional utility of these fluids. As an example, an additive package and a monitoring scheme that will control corrosion of a boiler condensate system, maintain the heat transfer performance, and/or extend the periods between maintenance shutdowns will have a direct beneficial economic impact on the overall system performance. In this regard, it is important to measure and monitor the content or effectiveness of various constituents of the aqueous based functional fluid during use and/or storage. Likewise, measurement of constituent concentration during manufacture is required to exercise quality control of the fluid produced.
The present invention relates generally to measurement of the surfactant concentration in aqueous working fluids. Anionic surfactants are prevalent in, and are often the preferred surfactant in aqueous working fluid composition and concentrated chemical additive packages for the treatment of aqueous systems. Given the wide use of surfactants in aqueous systems, a number of analytical techniques have been developed for determining the surfactant concentration of an aqueous working fluid.
Many of these prior art techniques have utilized two-phase titration techniques for measuring the anionic surfactant content. Two-phase titration techniques typically involve the titration of the anionic surfactant present in a known quantity of the aqueous working fluid (e.g., boiler water or chiller water) with a cationic titrant in the presence of a two-phase water/organic solvent (e.g., chloroform) system in the presence of an indicator. During the titration, a colored complex is formed in the aqueous phase and extracted into the organic solvent layer, signaling the endpoint of the titration. This same basic procedure is then repeated using a known quantity of one or more standards comprising an aqueous solution of the anionic surfactant having a known concentration. The amount of the aqueous cationic surfactant solution titrant necessary to reach the endpoint during the titration of the sample and standard solutions may then be used to calculate the amount of anionic surfactant present in the sample.
While performing such a two-phase titration procedure, it is necessary to shake or otherwise agitate the samples being titrated frequently in order to insure complete reaction between the surfactant and titrant and to promote the movement of the colored complex formed during the titration from the aqueous phase and into the organic solvent layer. It is also necessary to select an organic solvent that is essentially water insoluble and an appropriate indicator capable of forming a colored complex with the cationic titrant that is both essentially water insoluble and soluble in the organic solvent. It is especially preferred that the generally water-insoluble colored complex also have a color that may be easily distinguished from the color or colors assumed by the indicator when in water.
This two-phase titration procedure, while capable of producing generally accurate and repeatable concentration data, has several disadvantages. In particular, the two-phase technique is time consuming, requires frequent shaking of the samples, is dependent on the effectiveness of the shaking to obtain complete reaction, requires an organic solvent with the incumbent disposal problems and health concerns and may be relatively expensive.
In order to address at least some of these deficiencies, efforts have been made to develop an acceptable single-phase test technique. One such technique is disclosed in Ernst et al.'s U.S. Pat. No. 5,710,048, entitled “Determination of Surfactant Concentration in an Aqueous Fluid.” The single-phase technique disclosed by Ernst et al. comprises the steps of adjusting the pH of the test sample including an anionic surfactant to within a selected range of pH values, adding an indicator (toluidine blue) to the sample, adding a known amount of a standardized aqueous solution of 1,3-didecyl-2-methylimidazolium halide to the sample and indicator, and then titrating with a standardized aqueous solution of a polyvinylsulfuric acid alkali metal salt until a blue to pink color change is obtained. Once the amount of titrant needed is determined, the quantity of anionic surfactant in the original sample may be determined from one of a series of pH range specific standard curves. In this manner, Ernst et al. can provide a quantitative measurement of the anionic surfactant concentration without the need for a separated organic solvent phase and the inherent difficulties associated with using such a solvent. There remains a need, however, for simple, repeatable single-phase tests for determining the surfactant concentration in an aqueous system.